Electrical generators which are in the form of synchronous machines have one or more field windings on the rotor. A field winding is a conductor arrangement which produces a constant magnetic field during operation, which constant field rotates with the rotor, in order to induce an AC voltage in the windings of the stator. Brushes and sliprings may be provided in order to supply current to the rotating field winding. However, these are susceptible to wear. A so-called brushless exciter is therefore often provided, in which there is an auxiliary generator, which is in the form of an external-pole generator and supplies current to the field winding of the actual generator. In this auxiliary generator, a rotor winding which is fitted to the rotor rotates in a static magnetic field produced in the stator. The voltage which is produced in the rotor winding of the auxiliary generator is available, after appropriate rectification, for supplying the field winding of the actual generator. However, an auxiliary generator such as this requires a relatively large amount of space.
WO 95/26069 describes a rotating transformer for transmitting electrical power from the stator to the rotor, instead of an auxiliary generator. The rotating transformer described there comprises two coils which are wound concentrically around the rotation axis and are axially opposite each other along the rotation axis. This design allows the rotating transformer to be arranged only at the end of the rotor shaft, in practice, and this considerably restricts the design freedom, as a result of which this solution is particularly unsuitable when large generators have to be used.
U.S. Pat. No. 6,483,218 also describes a dynamoelectrical machine having a rotating transformer such as this. In this case, the transformer likewise comprises two concentric windings which are wound around the rotor axis, with the primary winding being arranged radially outside the secondary winding, and partially surrounding it. However, this arrangement also greatly restricts the possible forms of the dynamoelectrical machine. Furthermore, a very specific sequence must be complied with during assembly.
U.S. Pat. No. 3,758,845 describes a rotating transformer for an electrical machine, in which a flat, rectangular secondary winding is fitted on an outer surface of the rotor, and is radially opposite a corresponding primary winding. Although this transformer allows simple assembly and allows flexible forms for a dynamoelectrical machine equipped with it, the design means, however, that it is suitable only for the transmission of small signals. Furthermore, the power which can be transmitted is highly dependent on the instantaneous rotor position.